Since this root is pretty 'new' (November 2006), it might be that it is not in the set of default rootcertificates recognized by the Operating System.... To confirm, you could try with Firefox 8 or 9. It maintains it's own collection of trusted rootcertificates, so it will probably work because it's not dependent on the collection shipped in Windows Server.

If confirmed, there are the following possible solutions that I can think of.

i think it is possible to cross sign the *.wikipedia.org certificate to make it work with older collections of root certificates (not sure though if digicert still offers this feature....)